A flag which indicates whether any signal handlers will be installed during
startup. This includes handlers for SIGCHLD to monitor child processes,
and SIGINT, SIGTERM, and SIGBREAK to stop the reactor. (type: bool)

A flag which is true between the time reactor.stop is called
and the time the shutdown system event is fired. This is used to determine
whether that event should be fired after each iteration through the
mainloop. This should be replaced with an explicit state machine. (type: bool)

A flag which is true from the time reactor.run is called until
the time reactor.run returns. This is used to prevent calls
to reactor.run on a running reactor. This should be replaced
with an explicit state machine. (type: bool)